1. Field of the Invention
The present invention relates to an optical fiber transmission line for Raman amplification. More specifically, the present invention relates to a transmission line for Raman amplification and including a first fiber having a positive dispersion, a second fiber having a negative dispersion and a mode field diameter smaller than that of the first fiber, and a third fiber having a mode field diameter smaller than that of the second fiber.
2. Description of the Related Art
In long distance optical communication systems, an optical signal is transmitted through an optical transmission line. Moreover, wavelength division multiplexing (WDM) is currently being used in optical communication systems to increase transmission capacity. With WDM, two or more different wavelengths are multiplexed together and then transmitted together as a WDM optical signal through a single optical fiber as an optical transmission line.
Optical amplifiers are available which can amplify a WDM optical signal. Conventional optical communication systems typically combine the use of WDM and optical amplifiers to provide a high-capacity optical transmission system.
Moreover, optical regeneration repeaters are often positioned along the transmission line. An optical regeneration repeater converts the transmitted optical signal to an electric signal for retiming, reshaping and regenerating.
There are proposals to use an optical amplification repeater, which includes an optical amplifier, instead of using an optical regeneration repeater. By using an optical amplification repeater instead of an optical regeneration repeater, it is expected that the number of parts in the repeater can be significantly reduced, higher reliability can be achieved, and significant cost reduction can be realized.
FIG. 1 is a diagram illustrating a WDM optical communication system. Referring now to FIG. 1, an optical transmission station (OS) 1 includes a plurality of optical transmitters (E/O) 1A respectively outputting a plurality of optical signals of different wavelengths. The optical transmission station 1 also includes a multiplexer 1B wavelength multiplexing the plurality of optical signals together into a WDM optical signal, and a post amplifier 1C amplifying the WDM optical signal to a predetermined level and then outputting the amplified WDM optical signal to an optical transmission line 3.
An optical receiving station (OR) 2 receives the WDM optical signal from the transmission line 3. The optical receiving station 2 includes a pre-amplifier 2C amplifying the WDM optical signal. The optical receiving station 2 also includes a demultiplexer 2B demultiplexing the WDM optical signal into a plurality of optical signals depending on wavelength, and a plurality of optical receivers (O/E) 2A for respectively receiving the plurality of optical signals.
A plurality of optical repeaters 4 are arranged along the transmission line 3 at determined intervals. Each optical repeater 4 includes an optical amplifier for amplifying the WDM optical signal. In WDM optical communication systems, the optical amplifier is typically an erbium doped fiber amplifier (EDFA).
Raman amplification is also being used in WDM optical communication systems.
Moreover, optical communication systems have been proposed in which EDFAs are used in combination with Raman amplification, to thereby eliminate the use of optical regeneration repeaters.
With Raman amplification, a fiber is pumped with pump light in a manner which causes Raman amplification to occur in the fiber. Gain can be obtained in an inverse proportion to the mode field diameter of the fiber. Therefore, an optical fiber having a small mode field diameter is suitable for Raman amplification to provide a high gain.
For example, the mode field diameter of a negative dispersion fiber (−D fiber) having a 1.3 μm zero dispersion wavelength (referred to as a single mode fiber (SMF)) is about 5 mm. This mode field diameter is smaller than the mode field diameter of non-zero dispersion shifted fiber (referred to as a NZ-DSF), which is about 8 mm. Therefore, the SMF provides a relatively large Raman gain as compared to a NZ-DSF. See, for example, “Highly efficient distributed Raman amplification system in a zero-dispersion-flattened transmission line,” H. Kawakami et al., ThB5, 0AA'99, 1999.
For realization of a high capacity long distance optical communication system, the problems to be solved include insufficient optical signal to noise ratio (OSNR) per wavelength and transmission waveform distortion due to non-linear effect.
The OSNR per wavelength can be improved by the use of a distributed gain Raman amplifier. This type of Raman amplifier will enable optical amplification for a WDM optical signal including wavelengths over a wide band.
However, the use of a distributed gain Raman amplifier can cause transmission waveform distortion due to non-linear effect. Therefore, it is important to consider the non-linear effect of an optical fiber used for Raman amplification.
It is also important to consider the wavelength dispersion of the optical fiber used for Raman amplification.
Further, it is important to consider the size of a Raman amplifier, with a smaller size generally being more desirable. Particularly, when a Raman amplifier is adapted to an optical transmitting terminal station or optical receiving terminal station, it is desirable to shorten the length of optical fiber and reduce the size of an optical fiber module of the amplifier.